This invention relates to a molded closure for a container and more particularly to a method for molding closures having internal threads and removing of the threaded closures from a mold.
Closures made by molding with inner threaded features are well known in the art. Generally, the apparatus used for making these articles includes a mold arrangement having a mold cavity and a movable core. The moveable core is received within the cavity wherein a moldable plastic material therein is formed into a plastic article which is defined by the spacing between the walls of the cavity and the outer surface of the core. Upon completion of molding of the article, particularly a threaded cap or closure, the article is normally withdrawn from the cavity by the core. Thus, a mechanism is required which cooperates with the core for subsequently removing the threaded molded article therefrom. A number of different methods for removing the threaded article from the core are described in U.S. Pat. No. 5,421,717 to Hynds which teaches an apparatus for forming threaded molded articles.
It has been found that a mold cavity with at least one longitudinally extending rib on the inner molding surface of the cavity in cooperation with an axially and rotatably moving mold core element received within the cavity provides the means for ejecting a molded threaded article from the mold.
More particularly, the present invention provides a method for molding a variety of plastic parts. Specifically, the present invention provides a method of making a closure for a container comprising: inserting a preselected amount of a formable plastic material into a mold cavity having a selected first diameter, the mold cavity having at least one longitudinally extending rib along the walls thereof; moving a linearly and rotatably moveable threaded core into the cavity wherein the core is of a second diameter which is less than the first diameter thereby defining a spacing therebetween; contacting the plastic material and forcing the plastic material into the spacing defined between the walls of the cavity and the core thereby forming a closure having internal threads; and removing the core from the cavity by backing the core out and by simultaneously rotating the core either at selected intervals or continuously. Insertion of the preselected amount of formable plastic material can be done by placing said material into the opening in the mold before the core is inserted, thereby using the force of core insertion to mold. Alternatively, said insertion can occur after said core is inserted by use of a pin hole in said mold through which said material is injected under pressure. The longitudinally extending ribs prevent the rotation of the threaded article when the core is rotatably backed out of the cavity. When the core is linearly backed out without rotation, or backed out and rotated at any rate relative to the rise of the helix angle of the molded threads, the threaded portion of the core, being in cooperating relation with the threads of the closure, forces the closure to move out of the mold cavity. When the closure is completely free of the mold cavity, there is generally just a portion of an internal thread of the closure still engageable with the threaded core. Thus, the weight of a relatively heavy closure can cause the closure to fall away from the core. Alternatively, the molding core can have a concentric stripper ring or an ejector pin moveably connected, said ring or pin being in flush contact with the bottom of a newly-molded closure skirt at the completion of the molding step. The ring can act on the surface of the molded closure to strip the closure from the core. Likewise, the pin can act on the molded closure adjacent to the threaded opening on the side opposite the thread tail-out. Of particular benefit to those who work in the closure art is the fact that by use of both a mechanical stripping means such as a ring or a pin, and an axial separation coupled with relative rotation, the closure can be molded with buttress type threads which are optimized for plastic parts. Because a pin can be used to roll the cap off of the end of the core, buttress type threads are used for even the very last portion of thread which remains in contact with the core before the closure is separated from the core. The last portion of thread can be rounded in order to ease the distortion caused to said closure by the stripping process in designs incorporating concentric stripping.